This invention relates generally to bearing systems and, more particularly, to an apparatus and method for providing an axial preload on a bearing system.
One of the factors affecting bearing noise is the relative internal clearance within the bearing. The internal clearance of a deep groove ball bearing is typically expressed as the radial clearance between the inner raceway, balls, and outer raceway. Decreasing the internal clearance improves the contact between the balls and raceways. Decreased internal clearance also reduces the permissible temperature range for satisfactory operation. Machines expected to operate over wide temperature ranges must have increased internal clearance in the bearing compared to machines operating over relatively small temperature ranges.
Large internal clearances often produce increased noise and heat generation for bearings that have a large diameter or are for high speed operation, or both. The noise is the result of the balls moving into and out of the load region. When entering the load region the ball spin must match the linear velocity of the raceway. Upon leaving the load region, friction on the ball and cage will often slow the ball spin rate. The result can be an operating region where the ball momentarily skids or slides on the raceway when entering the load region of the bearing. This skidding or sliding often produces noise and additional heat generation within the bearing, leading to shortened bearing life. In bearings with ball guided cages, additional noise and heat may also be generated by the cage. Other factors affecting the skidding or sliding between the ball and raceway include lubricant viscosity, cage structure, and the precision of the balls and raceway surfaces.
An axial preload force is often recommended by the bearing manufacturer to maintain stable contact within the ball and raceway regions. The effect of the axial preload is to produce a small angle between the ball and raceway contact points and the rotating axis of the bearing. The angle is called the bearing contact angle. The axial preload also reduces the operating radial clearance within the bearing. The magnitude of the required axial preload force is a function of bearing size, speed range, lubricant viscosity, and loading. There is a minimum axial preload force that will effectively reduce bearing noise. Providing a consistent axial preload is often difficult due to tolerances and manufacturing processes. In order to provide a minimum preload at all times, a substantially larger preload often must be selected in order to account for the worst case system variability. In addition, a lower nominal preload force yields improved bearing life.
A common method for providing the axial preload force in a motor is to provide a spring pack outboard of one bearing, and provide a solid support for the outboard side of the opposite end bearing. In this configuration, the axial preload force is obtained by controlling the compressed height of the preload spring. The spring force is applied to the outer race of the bearing and is transmitted across the outer raceway and ball to the inner race through the contact angle. Similarly the force passes through the opposite end bearing from inner raceway to ball to outer raceway through the contact angle.
Additional forces may affect the axial preload applied to the bearing. Specifically, the preload spring end bearing will experience a force equal to the vector sum of the spring force and friction force occurring between the bearing outer race and support housing bore, and a sticking force occurring between the outer race and the support housing bore. Similarly, the bearing opposite the preload spring will be subjected to an axial force that is the vector sum of the preload spring force, the friction force, the sticking force, and any magnetic and application forces. Assuming that the axial components of magnetic and application forces are zero, the axial preload on either bearing is equal to the vector sum of the preload spring force, the friction force, and the sticking force.
Accordingly, since a minimum axial preload force reduces bearing noise and excessive force leads to shortened bearing life, it would be desirable to minimize the friction and sticking forces. Also, it would be desirable to simplify the fabrication process of the bearing system. Further, it would be desirable if the contact angle could be adjusted within the bearing.
These and other objects may be attained by a bearing system that includes a bearing housing with an opening and a bearing bore having a surface including a first relief cut and a second relief cut. In addition, the bearing system includes a bearing configured to support a rotatable shaft. The bearing is positioned within the bearing bore and includes an inner race, an outer race, and a plurality of balls. A preload spring is positioned adjacent the outer race and a first end of the spring exerts a preload force on the outer race. An adjust screw is placed in the housing opening and contacts a preload adjust plate that is positioned adjacent a second end of the preload spring. The adjust screw can be manipulated to increase and decrease the force exerted on the outer race by the spring.
The bearing outer race includes a first edge, a second edge, and a connecting portion. The bearing outer race is positioned in the bearing housing to allow the outer race first edge to be positioned at the first relief cut and the outer race second edge to be positioned at the second relief cut. The connecting portion of the outer race contacts the bearing bore surface while the first and second edges of the outer race do not contact the bearing bore surface.
A method for assembling the bearing system in an electric motor includes the steps of positioning the preload adjust plate, preload adjust screw, and the preload spring in the end shield assembly. The end shield assembly includes the end shield and the bearing housing. The bearing is positioned on a rotor shaft, and the inner race is press fit onto the shaft. The shaft is then inserted into the end shield assembly and is positioned to allow the first and second edges of the outer race to overlap the first and second relief cuts respectively. The relief cuts prevent the edges of the outer race from contacting the bearing bore during normal operation of the bearing system.
The preload spring is then adjusted to contact the outer race. The preload spring is adjusted by turning the adjust screw to provide a preload force to the spring that is then transmitted to the outer race of the bearing. The preload force moves the outer race to establish a contact angle within the bearing.
An alternative and simplified assembly can be constructed as previously described except that the pre-load adjust screws and plate are replaced by a shoulder or step machined into the bearing housing. The shoulder or step provides axial support for the pre-load spring, and the location of the machined shoulder or step is such that when the motor is fully assembled, the pre-load spring is compressed sufficiently to provide the desired pre-load force.
The combination of a preload spring acting on a bearing and the bearing bore including relief cuts is simple to manufacture since the relief cuts are relatively easy to machine and since the bearing bore surface does not need to have a polished surface. There is reduced edge loading of the bearing and reduced misalignment of the bearing that is normally caused by sticking of the outer race. In addition, sticking forces due to edge contact with the bearing bore surface are reduced and edge loading of the bearing is also reduced. Further, a more consistent axial preload force is applied to the bearing. Still further, by eliminating the contact of the bearing edges with the bearing bore, a more consistent surface contact is achieved during the various thermal cycles that the bearing system experiences during normal operation.